Most aircraft gas turbine engines, as well as axial flow turbomachines used in many other applications, have within them disks or rotors which carry a multiplicity of removable blades. Such structures are used in both the compressor and the turbine parts of the engine to respectively compress and expand the working fluid. In some instances the rotating blades have shrouds at their outermost tips and are connected at these locations. More commonly, modern engines have blades that lack shrouds and are only supported at their roots in the rotor disk. For high efficiency, it is desired to have the closest possible fit between the tips of the rotating blades and the sealing structure of the circumscribing case of the engine. The blades must be precisely machined to within as close as .+-.0.025 mm so that they are all at a constant radial distance from the center line of the engine. This presents a substantial machining problem, both in original part manufacture and in overhaul.
While the tolerances sought currently are tighter than previously, there has always been a desire to have bladed rotors fit well. Primarily this has been achieved by separately machining the rotors and blades to close tolerances and then assembling the parts. Preferably, horizontal rotary grinding machines have been used to machine the blades; fixtures rotating at a few hundred revolutions per minute hold the blades in a manner similar to that in which they are held in a rotor during use. Shims are placed under the blades to thrust them radially outward and to eliminate the inherent looseness related to the fit between blade and holder. However, such techniques have involved an accumulation of tolerances between the blades and rotor disk and they are inadequate to obtain the tolerances now desired.
Preferably, the rotor assembly for an engine is machined as a unit, as it will be used. This has been accomplished in one particular mode by the use of shims under the blades, as mentioned above, while spinning the rotor at no more than a few hundreds of revolutions per minute. Since most rotors used in gas turbine engines heretofore received individual blades in slots which ran generally perpendicular to the circumference of the part, the insertion of shims was relatively convenient. But, an alternative construction wherein the blades are received in a slot running circumferentially around the rim of the disk cannot be similarly treated. With this configuration it is not practical mechanically to insert shims. One approach believed taken by others has been to provide a close fit between the bottom of the blade root and the slot, thus preventing radially inward movement during machining. Sandwiching of the bladed rotor assembly between resilient pads has also been used to restrain the blade motion during grinding. But these methods do not appear capable of providing accuracies of better than .+-.0.075 mm.
A further problem has been that even though some relatively close fit tolerances were obtained previously, such tolerances did not necessarily result in the requisite high turbomachinery efficiency. It has been now found that part of the problem lies in the different seating dimensions assumed by the parts under the high rotational speed of use (of the order of 10,000 rpm), compared to the dimensions ascertainable from measurement of the static assembled parts. Also part of the problem resides in the fact that average dimensions of rotor assemblies were all that could be measured previously. Now, as described in commonly assigned Drinkuth et al patent application Ser. No. 501,982 "Method and Apparatus for Grinding Turbine Engine Rotor Assemblies Using Dynamic Optical Measurement System," apparatus is available for making individual blade measurements at high rotational speeds. The apparatus was useful in making the present invention.